In multipoint triangulation-based sensors, a series of aligned spots reflected from a workpiece are imaged by a camera that lies in the same plane as both the spots and the light sources.
The camera typically comprises a two-dimensional pixel array onto which the spots are imaged. The range to each of the spots is derived by triangulation from the location of its image on the array.
As was discussed in U.S. Pat. No. 5,986,745 to Hermary et al., a known limitation of such systems is the difficulty in sorting or discriminating between adjacent spots, for example as a result of the proximity or overlap of imaged spots, or due to dropout of a spot by occlusion from irregular surface characteristics.
One approach to enhancing reliable discrimination between spots is to more accurately determine the centroids of the spots along the triangulation axis. U.S. Pat. No. 5,056,922 to Cielo et al. discloses the use of an elliptical spot (e.g. the shape characteristic of TEM01 mode laser light) wherein the longitudinal axis of the elliptical spot is perpendicular to the longitudinal axis of the detecting elements of a linear array. This allows more accurate determination of the centroid of the spots and hence, better discrimination between adjacent spots.
U.S. Pat. No. 5,986,745 to Hermary et al. addresses the problem by using a spatially encoded pattern of light projected onto a workpiece rather than discrete spots.
U.S. Pat. No. 4,937,445 to Leong teaches the use of a small number of beams, so as to effectively limit the possibility of misidentification of a given spot, while U.S. Pat. No. 4,687,325 to Corby relies on a time-separated series of different patterns of beams.
It is apparent that the problem of spot discrimination in multipoint triangulation sensing systems is well known and that a variety of approaches are used to address it.
An object of the present invention is therefore to provide an improved, elegant and simple means of discriminating between successive spots in a multipoint triangulation-based sensor.
Optical triangulation sensors are often disposed on opposed sides of an object in order to characterize both sides of the object. In such cases, there is a risk of crosstalk between the two sensors when a projected spot is imaged on the opposite sensor enclosure rather than on the surface of the object to be measured. The present invention facilitates the avoidance of such crosstalk.
These and other objects of the invention will be better understood by reference to the detailed description of the preferred embodiment which follows.